CLIMATIC TRANSITIONS IN THE MEDITERRANEAN DURING THE DEVELOPMENT OF NORTHERN HEMISPHERIC GLACIATION

The Mediterranean Sea is an excellent region for paleoclimatic studies because its sensitivity to orbital forcing is enhanced by its geographic location, which allows it to be influenced by and to record climatic influences from the Atlantic Ocean and the three surrounding continents. Using the alkenone unsaturation ratio index (U_k37') on rock samples from three land outcrops from Sicily and southern Italy formed by uplifted and lithified sediments, we have compiled a sea-surface temperature record of the Mediterranean Sea from 3.65 to 1.56 Ma, with a sampling gap from 3.55 to 3.23 Ma. Our data show general trends of cooling and increasing temperature variability, as well as two distinct periods of climatic transition, at 2.58-2.43 Ma and at about 1.85 Ma (Plio-Pleistocene transition). Before 2.58 Ma, SST was relatively stable, varying in cycles of less than 3°C, driven primarily by local insolation. From 2.58 to 2.43 Ma, SST underwent two gradual phases of cooling, ending at a new minimum more than 1°C cooler than the last coldest point. From this transition onwards, SST was characterized by large, obliquity-driven cycles. From 2.43 to 1.85 Ma, SST varied in cycles of about 4.5°C. At ~1.85 Ma, coinciding with the Plio-Pleistocene transition, there was a significant two-fold increase in the amplitude of obliquity-forced SST cycles, with SST afterwards varying in cycles of about 9°C. This transition at ~1.85 Ma appears to be more pronounced than the corresponding intensification of climatic cycles found in other paleoclimatic records. Our results provide strong supporting evidence for the climatic transitions associated with Northern Hemispheric Glaciation that took place during the Plio-Pleistocene. Along with our SST record, we present a corresponding record of alkenone C37 total, which provides a record of bottom-water anoxia events caused by rhythmic, precession-driven variations of the African monsoon. These precession-paced anoxia events correspond to periods of sapropel deposition and facilitate orbital tuning in our age model. The application of the alkenone unsaturation proxy provides new quantitative data that both offer new insights into Mediterranean paleoclimate and support existing research that employed foraminiferal species ratios, isotopic data, and other proxies.